1. Field of the Invention
The present invention relates to transmission apparatuses and methods, reception apparatuses and methods, and programs. In particular, the present invention relates to a transmission apparatus and method, a reception apparatus and method, and a program which are capable of performing robust communication.
2. Description of the Related Art
As shown in FIG. 1, in typical wireless communication, a transmitter side multiplies an input baseband signal by a carrier wave and transmits the resulting radio signal (a modulation signal) to a receiving side.
The term “carrier wave (also referred to as a “carrier”)” refers to, in communication, an unmodulated, information-bearing reference signal transmitted through a transmission cable (wired communication) or electrical waves (wireless communication), generally, through waves (light, sound waves, or the like). Typically, varying the amplitude, the frequency, or the phase of the carrier wave makes it possible to transmit various types of information.
FIGS. 2A to 2C illustrate typical modulation schemes. ASK (amplitude shift keying) modulation shown in FIG. 2A is an amplitude modulation scheme in which information is conveyed by the amplitude. FSK (frequency shift keying) modulation shown in FIG. 2B is a frequency modulation scheme in which information is conveyed by the frequency, and PSK (phase shift keying) modulation shown in FIG. 2C is a phase modulation scheme in which information is carried by the phase.
In addition to the above-described modulation schemes, for example, modulation schemes in which phase modulation and amplitude modulation are combined are also generally used, such as QAM (quadrature amplitude modulation).
A radio signal modulated by such a modulation scheme is transmitted to the receiving side.
As shown in FIG. 3, the receiving side in the wireless communication re-multiplies the received radio signal (the modulation signal) by a carrier wave and extracts low-frequency components through a LPF (low pass filter) to thereby reproduce the transmitted signal. The carrier wave is synchronous in frequency and phase with the carrier wave used during the modulation.
In recent years, in environments where transmission channels are affected by frequency selective distortion, a multi-carrier scheme such as OFDM (orthogonal frequency division multiplexing) is typically used. OFDM is a special multi-carrier transmission scheme in which information is divided into multiple carrier waves for transmission.
Advantages and disadvantages of OFDM will now be described.
[Advantages of OFDM]    (1) It is robust against frequency selective fading in a multipath carrier channel, since a large number of sub-carrier waves (i.e., information transmission channels for respective carriers) are used.    (2) Frequency interleaving can be performed in addition to time interleaving and the effect of error correction can be effectively used.    (3) The symbol period is long, and provision of a guard interval (GI) can reduce interference due to reflection waves.    (4) Since the carrier waves in OFDM have low bit rates and are digitally modulated waves in a narrow band, the spectra of sub-channels can be closely arranged and the frequency utilization efficiency is high.    (5) It is possible to perform flexible information transmission, for example, without using a channel that is assumed to encounter interference.    (6) Information can be easily hierarchized, for example, by changing a modulation scheme for the sub carrier waves.
[Disadvantages of OFDM]    (1) Because of multi carriers, characteristic deterioration due to mutual modulation occurs when the transmission channel has a nonlinear characteristic.    (2) The transmitter and the receiver become complicated in order to maintain orthogonality of many carriers. In particular, the receiver may necessitate a special arrangement so that signals are correctly reproduced even if the transmission-channel characteristics vary.    (3) It is difficult to achieve symbol synchronization.    (4) Frequency offset causes characteristics between carriers to vary and deteriorate.    (5) Since an advantage is obtained by using a larger number of carriers, a large bandwidth in which a large number of channels are multiplexed is typically necessary even for audio transmission.    (6) A high-rate A/D (analog/digital) converter is typically necessary.
As a technology associated with OFDM, for example, Japanese Examined Patent Application Publication No. 60-13344 discloses a related technology.